1. Field of the Invention
The present invention relates to the technical field of face recognition and, more particularly, to an iris extraction method.
2. Description of Related Art
Currently, in the technical field of face recognition used for analyzing and determining digital face images, the basic requirement is to determine the position or range of eyes in an image so as to continue subsequent processes of recognition. For example, in a full-face recognition, a rough position of eyes is required for aligning an untested image and a reference image. While with the development of information technology, the desired precision of recognition is getting stricter. For example, the conventional recognition method used for determining the direction of the user's sight needs not only the correct position of the center of eyes, but also the exact location of irises as an assistant. With reference to FIG. 1, there is shown an eye image 1. The annular area around the pupil 11 is an iris 12. The color of iris 12 differs from each other, which may be black, green, blue, or brown. The white area exposed in the surface of eyeball is a sclera 13.
Compared with skin color, irises are circular dark areas in a face image. Therefore, the most popular conventional methods used for extracting irises in the face image include the circle Hough transform (CHT) method, and the deformable template match (DTM) method.
In the conventional method, at first, two searching regions are roughly defined as a pair of eyes in an input image. Next, a predefined energy function is used for measuring the energy of each pixel in the searching region. Each pixel is taken as a center of a circle with a radius ri (i=1˜n, rn is the maximal radius, ri is the minimal radius), so as to build a circular deformable template. The energy function can be expressed as follows:
      Eg    =                  1                            C                              =                                                C                    ⁢                      ϕ            ⁡                          (              S              )                                ⁢                                          ⁢                      ⅆ            s                          +                              1                                        A                                              ⁢                      ∫                                          ∫                A                                                                              ⁢                                                ψ                  ⁡                                      (                    a                    )                                                  ⁢                                  ⅆ                  x                                ⁢                                  ⅆ                  y                                                                          ,where φ(S) is an edge intensity of one of the pixels in the periphery of the circular template, ψ(a) is a gray value of the pixel of input image within the range of the circular template. Usually, an original inputted gray-value image is required to obtain an edge map with the distribution of edge intensity by performing a contract operation, such as Sobel operators. Since a pixel has n energies due to n different sizes (radiuses) of circular templates, the maximal energy of a pixel is regarded as the energy of the pixel. Then, two pixels are respectively selected from two searching regions as positions of irises so as to finish the iris-positioning procedure, where each pixel is the one with the maximal energy of a plurality of pixels in the searching region.
However, generally, irises are partially visible in an input image in reality. Even though when the eyes look forward, only two-third irises are visible. Further, in most situations, eyelids may cover upper irises, especially when the eyeball is rotating or the eyes are looking sideways. Also, the uncertainty (e.g. uneven light) of photo environment and the local circular-like dark areas of surrounding objects (e.g. eyebrows, eyeglasses, and hair) may lead to misjudge the eyebrows or glasses as best positions of irises according to conventional method. Therefore, it is desirable to provide an improved iris extraction method to mitigate and/or obviate the aforementioned problems.